Feedback mechanisms in monsoon transitions

Recent studies (Schneider and Bordoni, 2008; Bordoni and Schneider 2008) have shown that rapid monsoon-like transitions can occur in idealized numerical experiments even in the absence of land-sea contrasts, provided the surface has sufficiently low thermal inertia. In these simulations, monsoon transitions are mainly driven and are rendered rapid by dynamical feedbacks involving large-scale midlatitude waves and the tropical circulation. Other studies (Boos and Emanuel, 2008a,b) point to the importance of moisture-dynamic feedbacks, such as wind-induced surface heat exchange (WISHE), in the rapid onset of monsoon systems.

Here, we use an idealized aquaplanet GCM with an active hydrological cycle to explore the role that each of these feedbacks plays in the dynamics of monsoon transitions. Results from a control run, in which large-scale eddies are resolved and the WISHE feedback is active, are compared to runs in which the large-scale eddies and the WISHE feedback are separately or simultaneously suppressed. It is found that rapid monsoon transitions do not occur when the tropical circulation approaches the nearly inviscid limit throughout the year, as is the case when large-scale eddies are suppressed. Suppressing the WISHE feedback, however, has a smaller impact on the simulated monsoon, in that monsoon transitions similar to those seen in the control run still occur. Sensitivity to changes in the surface thermal inertia (through changes in the mixed layer depth) is discussed.